1. Field of the Invention
The present invention relates to display apparatus, and in particular, to display apparatus including display elements for displaying a plurality of digits.
2. Description of the Prior Art
Generally, domestic electrical appliances such as microwave ovens include display devices which display, for example, the remaining time for cooking or the temperature of food during a cooking operation. These display devices are provided with a number of display elements, each of which is formed in a segment arrangement with light-emitting diodes. The driving of the display elements may be multiplexed so that the number of signal wires between each display element and a drive voltage source can be reduced. Various types of the above-described prior art will be described in more detail with reference to FIGS. 1 to 3.
FIG. 1 shows a static drive type display apparatus. A display device 1 includes display elements D.sub.1, D.sub.2, D.sub.3 and D.sub.4, each of which is formed with light-emitting diodes. Each one of the terminals (digit terminals) of individual display elements D.sub.1, D.sub.2, D.sub.3 and D.sub.4 is connected to the secondary side of a transformer 3 through parallel connected diodes 5 and 7. Other terminals (segment terminals) thereof are connected to a microcomputer 9 individually. An input of microcomputer 9 is connected to center tap 11 of transformer 3, whose primary side is connected to commercial voltage supply 13. Display elements D.sub.1, D.sub.2, D.sub.3 and D.sub.4 are sequentially switched on and off by microcomputer 9. In other words, display elements D.sub.1, D.sub.2, D.sub.3 and D.sub.4 are supplied, in turn, with a prescribed D.C. voltage which is produced by transforming and full-wave rectifying the commercial voltage of voltage supply 13.
FIG. 2 shows a duplex drive type display apparatus, in which one of the terminals (digit terminals) of individual display elements D.sub.1 and D.sub.3 are connected in parallel to one another and connection point 15a thereof is connected to one of the secondary side terminals of transformer 3 through diode 5. One of the terminals (digit terminals) of individual display elements D.sub.2 and D.sub.4 are also connected in parallel to one another and the connection point 15b thereof is connected to another secondary side terminal of transformer 3 through diode 7. Other terminals (segment terminals) of display element D.sub.1 are connected in parallel to corresponding terminals of display element D.sub.2, respectively, and the individual connection points are connected to microcomputer 9. In the same manner, corresponding other terminals (segment terminals) of display elements D.sub.3 and D.sub.4 are connected in parallel and each connection point is connected to microcomputer 9 individually. Thus, the positive half-wave voltage of power supply 13, obtained through diode 5, is supplied to display elements D.sub.1 and D.sub.3, and the negative half-wave voltage thereof, obtained through diode 7, is supplied to display elements D.sub.2 and D.sub.4. The operation of display elements D.sub.1 and D.sub.3 and the operation of display elements D.sub.2 and D.sub.4 are carried out in parallel by microcomputer 9.
In the above-described display apparatus, however, there is a drawback that ripple components in the voltage of the commercial voltage supply 13 cause flickering on display device 1, and spoils its display.
To solve the above drawback, in a dynamic drive (multiplex) type display apparatus as shown in FIG. 3, the operating voltage for display device 1 is obtained from a stabilized DC power supply 17. In FIG. 3, one set of terminals of individual display elements D.sub.1, D.sub.2, D.sub.3 and D.sub.4 is connected to microcomputer 9 through a digit driver 18, and other corresponding sets of terminals for the display elements are connected in parallel and the individual connection points are connected to microcomputer 9 through a segment driver 19. Stabilized DC power supply 17, obtained by rectifying and stabilizing the commercial voltage, is connected between digit driver 18 and segment driver 19. With this arrangement, since the ripple components in the commercial voltage can be eliminated, it is possible to achieve a stable display.
However, the stabilized DC power supply provided to prevent display flickering causes some new problems by complicating the circuit arrangements and increasing production costs. In addition, there is also a problem of large power losses in the stabilized DC power supply.